#app {
  display: flex;
  flex-direction: column;
  align-items: center;
}

header {
  width: 100%;
  height: 40px;
  background-color: #555;
  display: flex;
  justify-content: center;
  align-items: center;
}

.headerCenter {
  width: 1200px;
  height: 40px;
  background-color: orange;
  display: flex;
  justify-content: space-between;
}

nav {
  width: 1200px;
  height: 140px;
  background-color: aquamarine;
}